No. 602] MENDELIAN FACTOR DIFFERENCES 93 



et al, I. c), presumably the male in tMs case, then it would 

 be possible in the male cavy to secure only recombinations 

 involving whole chromosomes, or in other words involv- 

 ing the building up of reaction systems with a large num- 

 ber of interchanged factors. In the female, however, 

 these recombinations might include, in addition to those 

 resulting from redistribution of whole chromosomes, 

 cross-over gametes resulting from the exchange of sec- 

 tions of chromosomes. Such gametes might conceivably 

 contain fewer discordant elements or they might be dis- 

 tributed in such a way as to distrub the reaction system 

 thus formed less profoundly. Such data of course need 

 first to be reexamined from this viewpoint before any 

 definite conclusions can be reached, but the list of species 

 hybrids which Detlefsen gives which result in sterile 

 males and fertile females would seem to indicate that this 

 is a phenomenon connected in some way with the ob- 

 served lack of crossing-over in the sex-heterozygote 

 already demonstrated in the work with Drosophila and 

 the silkworm (Sturtevant, 1915). Until we know more 

 about the fimdamental basis of crossing-over and the 

 factors at¥ecting it, it is idle to speculate on such differ- 

 ences in behavior as are shown by males and females in 

 the species crosses mentioned. 



The insistent way in which the species hybrids between 

 Tabacum and sylvestris point to the conception of the 

 Mendelian reaction systems as units in themselves is of 

 interest because of the broad and far reaching conse- 

 quences which follow the application of such an idea. For 

 if the nature of the progeny of these partially sterile 

 hybrids as grown through several generations points to 

 anything, it is that the abortive ovules and pollen grains 

 represent a selective elimination of certain types of re- 

 combinations. Obviously, then, the presence of any con- 

 siderable proportion of sterile ovules or pollen grains in 

 plant material may be a consequence of hybridity and 

 that of a rather profound type involving reaction systems 

 which are more or less specific in their nature and in part 

 incompatible with each other. The importance of this 



